Recently, in relation to thin film crystal growth technologies for compound semiconductors, semiconductor devices using compound semiconductor substrates have been developed markedly while exhibiting various characteristics. Examples of such semiconductor devices include: electronic devices such as an HEMT (high electron mobility transistor) and an HBT (heterojunction bipolar transistor); optical devices; solar cells; ultrahigh-speed devices using two-dimensional electron gas; magnetic sensors; and the like.
However, it is difficult to grow a large crystal for a compound semiconductor substrate. Moreover, since a compound semiconductor substrate is brittle and easily broken, care is required in treating the compound semiconductor substrate during a manufacturing process. Meanwhile, since a compound semiconductor is expensive, transition to a compound semiconductor substrate having a large diameter is necessary. However, a compound semiconductor substrate having a large diameter is heavy and brittle, and hence there is a concern about the yield.
In this connection, attention has been focused on technologies for forming a compound semiconductor on a group IV semiconductor substrate which is inexpensive, excellent in crystallinity, light in weight, and suitable for achieving a larger diameter. In particular, research has been actively conducted on technologies for growing thin film crystals of GaAs, which is a compound semiconductor, on a Si substrate for which a production technology has been established. Additionally, in the case of using a compound semiconductor substrate in a device, characteristic distribution over an entirety of the compound semiconductor substrate needs to be narrow.
However, various problems arise when epitaxial growth is performed on a Si substrate. This is because a crystal of Si and a crystal of a compound semiconductor have different lattice constants and coefficients of thermal expansion, and hence misfit occurs therebetween. For example, the lattice constants of Si and GaAs differ by 4%, and there is a two-fold difference between the coefficients of thermal expansion of Si and GaAs. Under such conditions, it is not easy to perform epitaxial growth on a Si substrate.
Moreover, regarding a surface treatment of the Si substrate, it is important to remove SiO2, which is an oxide on the Si substrate, before the epitaxial growth of a compound semiconductor.
In a first method for removing the oxide, the oxide can be removed by raising the temperature of the Si substrate under high vacuum. From an industrial perspective, however, the first method has poor mass-productivity because of an inferior TAT thereof.
As a second method for removing the oxide, there is a method for removing the oxide by chemically treating the Si substrate. In the second method, the oxide is removed by treating the Si substrate with hydrofluoric acid before the Si substrate is placed in a film formation apparatus. Moreover, a surface of the Si substrate is protected in a hydrogen-terminated state, whereby oxidation after the treatment is prevented. In the second method, initial growth which controls release of termination hydrogen is also necessary.
As an example of a conventional industrial technique for growing a group III-V compound semiconductor on a hydrogen-terminated Si substrate, Patent Document 1 discloses a method, regarding InSb, for producing a hetero-epitaxial film. In this method, two-stage growth is performed by forming an under-layer of at least any one of aluminum, gallium, and indium in initial growth on a Si substrate of two-stage growth. However, a high quality GaAs film cannot be obtained by a similar method.
Particularly, when a compound semiconductor having a different lattice constant or a different coefficient of thermal expansion is grown on a Si substrate, many dislocations and lamination stacking faults are created at an interface between Si and a compound semiconductor layer, even with a two-stage or three-stage growth process being employed. Such dislocations (defects) inherent in an interface cause deterioration in quality of electrical characteristics of a compound semiconductor which serves as an active layer of an electronic device. Hence, electrical characteristics expected for an unimpaired compound semiconductor cannot be obtained. When a current flows in a direction perpendicular to a substrate face, for example, in an optical device, dislocations (defects) at the interface also influence the decrease in luminous efficacy.
Meanwhile, when an electronic device is fabricated by use of a compound semiconductor substrate obtained through hetero-epitaxial growth on Si, dislocations (defects) present at an interface, which are created in an initial stage of hetero-epitaxial growth process, affect characteristics of the above-described electronic device. This is because such dislocations (defects) are considered as one of factors that cause dislocations (defects) appearing in a surface of the compound semiconductor substrate.
For example, in a case of a Hall element, Patent Document 2 describes a fact that dislocations (defects) of a GaAs substrate lead to the worsening of an unbalanced (offset) voltage, which is an output voltage under no magnetic field, and discloses that the dislocations are improved by epitaxially growing GaAs on a GaAs substrate, whereby the unbalanced (offset) voltage is reduced.
Meanwhile, Patent Document 3 discloses a light emitting device which is, by using a substrate having low dislocations (defects), high in luminous efficacy and less likely to deteriorate.
Regarding crystal dislocations (defects), Non-Patent Document 1 discloses lateral growth as a growth method with which crystal dislocations (defects) are improved. The lateral growth achieves a local improvement in crystal dislocations (defects), and hence it is difficult to obtain a substrate having a high crystallinity over the entire surface of the substrate used. Moreover, there is a drawback that pretreatment of a substrate is complicated. When electronic devices, optical devices, magnetic sensors, or the like are produced in quantity by use of a compound semiconductor substrate, it is necessary to improve crystal dislocations (defects) over the entire surface of the substrate used, and thereby achieve a good yield.
However, in the above-described Patent Document 1, dislocations (defects) at the interface between the Si substrate and the compound semiconductor layer are not sufficiently reduced by the initial growth. Since the compound semiconductor layer is formed in a film thickness of 4.0 μm, the electrical characteristics influenced by defects created at the interface are seemingly improved. However, since an epitaxial layer as thick as 4.0 μm is necessary, the dislocations (defects) in the compound semiconductor layer are not improved. Moreover, GaAs films, which have a higher crystal growth temperature, having a quality enough for use in devices such as Hall elements have not been obtained industrially. In a case where a compound semiconductor is hetero-epitaxially grown on a Si substrate, an initial stage in a hetero-epitaxial growth process is most important.
The present invention has been made in view of such circumstances, and an object of the present invention is to provide: a compound semiconductor substrate which has, in a compound semiconductor layer on a Si substrate, a reduced dislocation (defect) density at any location of an interface between the Si substrate and the compound semiconductor layer, and has a large surface area of 10 cm2 or more; and a method for producing the compound semiconductor substrate.
Another object of the present invention is to provide a semiconductor device using the compound semiconductor substrate, the semiconductor device being most suitable as an electronic device such as an HEMT or an HBT, an optical device, a solar cell, an ultrahigh-speed device using two-dimensional electron gas, or a magnetic sensor.
Patent Document 1: Japanese Patent Laid-Open No. Hei 7-249577
Patent Document 2: Japanese Patent Laid-Open No. Hei 1-95577
Patent Document 3: Japanese Patent Laid-Open No. Hei 7-193331
Non-Patent Document 1: D. Pribat et al., Jpn. J. Appl. Phys. 30, L431 (1991)